This invention relates to an improved process for spinning acrylic fibers from solutions of particularly high polymer concentration. The resulting fibers have improved initial whiteness as well as improved whiteness retention on heating.
Polymers for the preparation of acrylic fibers, which by definition contain 85% or more by weight acrylonitrile, are ordinarily prepared as an aqueous slurry using redox catalysts, e.g., potassium persulfate initiator and sodium bisulfite activator. In fiber form, these polymers have the disadvantage of being somewhat off-white in color as formed and discolor even further on heating at high temperatures. It is known that initial yellowness (lack of whiteness) and the tendency to discolor further on heating of the acrylic polymers is inversely related to the polymer molecular weight. Therefore, manufacturing practice has been to adjust polymer molecular weight to that required to provide fibers of acceptable whiteness. The use of higher molecular weight polymer than is needed to provide adequate fiber physical properties results in a loss of productivity since the solutions used in processing such polymers have higher viscosities than would otherwise be needed.
While the source of yellowness in acrylonitrile polymers and fibers prepared therefrom is not completely understood, it is now generally accepted that the color is due to a chromophoric structure consisting of a series of condensed naphthyridine rings each bearing a ##STR1## residue, several of which in an unbroken series absorb in the ultraviolet region of the spectrum, rendering the polymer yellow.
One method proposed for blocking formation of this chromophore is to prepare copolymers wherein the acrylonitrile units are separated by copolymeric units sufficiently often to prevent aggregation of the six or seven consecutive acrylonitrile units required for color formation. While effective, this method is generally not useful in the case of fibers because the amount of comonomer required, e.g., about 21% by weight in the case of methyl acrylate, is not conducive to good fiber properties, especially with respect to dimensional stability. Bulky comonomers are more effective on a weight percent basis in preventing formation of the chromophore but are equally disadvantageous with respect to dimensional stability. For example, as little as 10.5 weight percent styrene copolymerized with 89.5% by weight acrylonitrile results in significant shrinkage of fibers prepared therefrom under the hot-wet conditions encountered in commercial dyeing and laundering. Most commercial acrylic fibers contain no more than 9% by weight comonomer(s).
It has recently been proposed by Brandrup, Peebles et al., Makromol. Chem., 98, 189 (1966) and Macromolecules, 1, 53-8 (1968) that the naphthyridine chromophores are formed from .beta.-ketonitrile groups derived from an adduct formed by free radical attack on the nitrile groups in the polymer. U.S. Pat. No. 3,448,092 (Chiang) describes a polymerization process using coordination catalysts which provides acrylonitrile polymers having less than 5 .mu.eg./g. .beta.-ketonitrile groups. These polymers have improved stability to discoloration on heating. However, this process is disadvantageous because non-aqueous solvents must be used.
U.S. Pat. No. 3,828,013 (Nield) describes an emulsion polymerization process for preparing acrylonitrile polymers containing up to 95 mol percent acrylonitrile (90.6% acrylonitrile by weight when copolymerized with styrene) using a combination of low volatility and high volatility mercaptans as chain transfer agents to control molecular weight. Although primarily intended for the molding of bottles, the polymers are also said to be suitable for the preparation of fibers. Color stability of the polymers on heating is not mentioned.
Another emulsion polymerization process for the preparation of acrylonitrile polymers is described in U.S. Pat. No. 3,819,762 (Howe). Dodecyl mercaptan is used as a chain transfer agent in some of the examples but is not required by the claims. The resulting polymers containing up to 85% by weight acrylonitrile are suitable for molding into bottles. No suggestion is made that the polymers are suitable for the spinning of fibers.
The present invention provides an improved solution spinning process for the preparation of acrylic fibers having the process advantages of higher feasible polymer concentration, reduced sensitivity to discoloration resulting from process interruptions and reduced solution viscosity at the same polymer intrinsic viscosity.
This invention provides an improved process for spinning acrylonitrile polymer fibers from an acrylonitrile polymer containing at least 91% by weight acrylonitrile units and up to 9% by weight copolymeric units having an intrinsic viscosity of 0.6 to 2.0, 7 to 23 .mu.eq./g. enolizable groups after mild acid treatment, 15 to 70 .mu.eq./g. thioether ends derived from a water insoluble mercaptan and less than 3 .mu.eq./g. oxidizable hydrolysis fragments wherein the polymer is dissolved in a solvent for the polymer to provide a solution having a polymer concentration of 38 to 75% by weight which is then spun by conventional spinning methods. Preferably the polymer concentration is 40-50% by weight and the intrinsic viscosity is 0.8 to 1.5. Most preferably the intrinsic viscosity is 0.9 to 1.1 and the solution is dry spun. Wet spinning is also preferred. Spinning may also be accomplished by plasticized melt spinning.
Polymer suitable for use in the present invention may be conveniently prepared as an aqueous emulsion using water, the desired monomers, relatively low concentrations of a free radical initiator, a surfactant and a water insoluble mercaptan as chain transfer agent. The resulting latex may be coagulated by any convenient means to facilitate isolation of the polymer.
The initiator may be a persulfate acid or salt such as potassium persulfate, an azo initiator such as azo-bis(isobutyronitrile), azo-bis-(.alpha.,.alpha.- dimethylvaleronitrile) or azo-bis(.alpha.,.alpha.-dimethyl-.gamma.-methoxyvaleronitrile) or a peroxide initiator such as t-butyl peroxyneodecanoate or other free radical initiator known in the art.
Low radical concentration is achieved by using a low initiator concentration and operating at low monomer(s)/H.sub.2 O ratio and at temperatures as low as consistent with satisfactory conversion and yield. Usually polymerization in emulsion gives whiter, more stable polymer than polymerization in suspension probably because the polymer accumulates in the nonaqueous phase and thus is insulated from attack by radicals which are formed in the aqueous phase from the water soluble initiator (persulfate). The dodecyl mercaptan or other thiol chain transfer agent serves a dual function. It controls molecular weight by endcapping growing polymer radicals with hydrogen while initiating another chain with the residual RS.radical. Not only is the hydrogen capped end of the first chain stable but also the thioether end of the new chain is highly stable. Thus the second function is to supply a preponderance of stable ends.
The mercaptan chain transfer agent should be essentially insoluble in water. Aliphatic mercaptans having more than 7 carbon atoms are essentially insoluble in water. Dodecyl mercaptan is preferred. Use of an essentially water insoluble mercaptan made available in the polymerization zone by addition of a mutual solvent or an effective emulsifier tends not only to increase the resistance of the polymer to discoloration but also to compensate for the lower polymerization rate entailed by using a low initiator concentration.
Although dodecyl mercaptan is the preferred chain transfer agent, other oil soluble mercaptans including alkyl or aralkyl mercaptans varying in carbon atoms per molecule from 6 to 20 or more may be used. Other nonreactive groups such as hydroxyls, ethers and esters may be present so long as they do not increase water solubility and decrease oil solubility greatly. A final consideration is that the shorter chain mercaptans of C.sub.8 or C.sub.6 carbon content typically give lower polymer yields than do longer chain mercaptans.
Suitable surfactants should be nonsubstantive on the polymer, i.e., other than cationic if the polymer is designed to be dyeable with cationic dyes. Approximately 5% by weight or less of this surfactant, based on monomers, should efficiently disperse the monomers and chain transfer agent and provide an emulsion of the polymer that is coagulable yet stable to monomer stripping conditions and storage. Preferably, the surfactant should be removable by washing with water. Alkylphenol polyethyleneoxy sodium sulfates having up to 10 ethyleneoxy groups are preferred. The corresponding phosphates are also useful but are more difficult to remove because of lower solubility in hot water. In most instances, at least 0.5% by weight surfactant is required.
The amount of agitation required to produce the acrylic polymers useful in the present invention depends on the composition of the polymerization medium. If a preferred surfactant is present in sufficient quantities to provide a stable emulsion of the polymer, moderate agitation is sufficient. However, more vigorous agitation is required with use of lesser amounts of surfactant or with use of a less efficient surfactant. A deficiency in agitation can be compensated for in part by an increase in mercaptan content. Likewise, increased agitation tends to reduce the amount of mercaptan required to provide a given molecular weight polymer, other factors being constant.
The polymerization preferably is carried out in the range of 25.degree.-65.degree. C. Use of relatively high temperatures increases the rate of polymerization while reducing the molecular weight of the acrylic polymer. Use of relatively low temperatures has the opposite effect. Use of temperatures below about 25.degree. C. results in polymerization rates too low to be commercially useful while temperatures above 65.degree. C. encourage inefficient initiator decomposition and increase side reactions between the initiator and the mercaptan chain transfer agent.
Polymer may be recovered from emulsions by freezing or coagulation of the latex with salts or acids. Preferably, excess monomers first are stripped off under vacuum to prevent further polymerization and to facilitate coagulation. Salts such as sodium chloride, aluminum sulfate or magnesium sulfate and acids such as hydrochloric, sulfuric or phosphoric acids are useful coagulants. After the coagulant is added to the stripped latex, the mixture is heated until the coagulated particles grow large enough to filter easily.
Spinning solutions containing 40% by weight acrylonitrile polymer can be readily prepared using conventional mixing equipment. Solutions of higher concentrations, e.g., about 50% by weight acrylic polymer or higher are best prepared using twin screw extruders such as described in U.S. Pat. No. 4,028,302 (Tynan).
Spinning may be accomplished under conventional wet and dry spinning conditions. However, the improved stability of the polymer useful in the present invention allows use of much higher process temperatures without excessive discoloration of the resulting fibers. For example, use of cell wall temperatures as high as 280.degree. C. in the dry spinning of acrylic fibers from N,N-dimethylformamide (DMF) solvent gave fibers of good color having a DMF content of only 0.7% as compared to 17% obtained when the usual cell wall temperatures of 140.degree. C. are used.
By adding an optical brightener and a toner, acrylic fibers having whiteness similar to that of bleached cotton are obtained. Suitable brighteners are, for example, the coumarins taught in U.S. Pat. Nos. 2,945,033 and 2,878,138, the stilbenes taught in U.S. Pat. No. 2,838,504 and the p-substituted amidobenzoyl derivatives taught in U.S. Pat. No. 2,911,415.
Suitable toners are dyes, pigments or combinations of two or more dyes and/or pigments that are stable and retained under conditions of fiber manufacture and use. They should be suitably complementary in color to the yellowness of the fiber. Examples are Color Index Pigment Blue 10 or 15 and Color Index Pigment Violet 5.
The acrylic polymers useful in the present invention have lower solution viscosities at a given polymer concentration than comparable solutions of conventional slurry redox acrylic polymer. This is believed to be due to a lower degree of chain branching in the polymers useful in the present invention. The differences are especially noticeable at concentrations greater than 38% by weight.